Mario Tennis Open is a sports game for the Nintendo 3DS and the sixth (fifth in PAL regions) installment of the Mario Tennis series (developed by Camelot Software Planning). The game features gyroscope support and online multiplayer (using the Nintendo Network procedures). This is the first handheld installment in the series to not include a Story Mode or RPG elements and the first handheld installment with a Tournament mode. This game is also the first Mario game to use QR (Quick Response) codes. In this case, it is used to unlock playable characters and the Yoshi costume.

The gameplay features the traditional basic elements from previous Mario Tennis games. To win, the player must score points by hitting the ball into the other side of the court and bounce twice, the basic objective of tennis. Players earn 15 points for every shot that is successful and can win the game by earning set, game, and match points by earning 60 points on each game. The number of sets and games can be changed by the player in exhibition mode but not in tournament mode.

This game uses the buttons of the 3DS during single or multiplayer matches, though players can perform various tennis shots by selecting the shot panels on the touch screen, which will light up to alert the player of the best shot to use in a given situation. By holding the 3DS vertically, players can make use of the aforementioned gyroscope support, disabling 3D functionality. This places the camera behind their character, whose movement becomes automatic, and allows players to control the direction of their shots based on the position of the console. The gyroscope support can be disabled either by holding the console horizontally or by disabling it in the Options menu. There are also Special Modes in the game, such as "Super Mario Tennis", where the player needs to hit enemies, blocks and coins with tennis balls in some levels of Super Mario Bros..

- During a match, this button pauses the game and opens a menu that contains choices such as viewing game rules, setting gyroscope controls, choosing to re-do matches, and canceling matches.

- Toggles between the character's dominant hand.

- Toggles between the character's star rank. In multiplayer, this is based on the data of player who created the room. Therefore, the player who has created the room needs to have his or her characters starred if members of the room want to give their characters a star rank.

Chance Shots can appear if the opponent performs a bad rebound. In that case, a small colored area with a symbol of a Mario enemy or item appears in the player's court. The color of the symbol matches the colors of the panels in the touch screen (if the touch screen is set to the 6-panel shot panel). To perform a chance shot, players have to perform a shot whose color corresponds to the colored area when they are in that spot, either by pressing the correct button or button combination or by touching the matching color on the touch screen. Otherwise, the shot is a normal one. However, players can perform a simple shot that automatically selects the appropriate shot, but this Chance Shot is slightly weaker.

Players on the receiving end of a Chance Shot experience special effects that can hinder them. However, they can lessen the effect by pressing the opposite shot. For instance, red Chance Shots may not make much of an impact if the player retaliates with a (blue) slice shot. The recommended counter button is lit up for the receiving player.

Tournament mode is very similar to that in the previous Mario Tennis games. However, two more cups are added and are arranged differently. Players must have a star ranking to participate in the second set of cups. To do that, they must beat the Champions Cup. However, in the Doubles Tournament, only the character they control earns the star rank. Beating Champions Cup unlocks the Pro difficulty, which is more difficult than Expert, while Final Cup unlocks Ace, the most difficult COM level for Exhibition Mode.

Once the player has unlocked a cup, they can play it at any time, using any character. As a result, characters do not have to clear all three cups to beat the Champions Cup to earn the Star Rank, unlike in previous titles. Like in previous titles, though, computer opponents will not use the hidden characters in Tournament.

Similar to the preceding Mario Tennis games, exhibition mode is a basic versus mode. Players can choose a singles or doubles match. After that, they can choose their character (and teammate, for doubles) and opponents and press the or to give characters a left-handed dominance or a star rank, respectively, if they want. The opponent's CPU's difficulty can be chosen after that, ranking from lowest to highest: Novice (blue triangle), Intermediate (yellow circle), Expert (green circle with dot in the middle), Pro (red diamond), and Ace (rainbow star). The Pro and Ace difficulties are unlockable by winning the Champions Cup and Final Cup, respectively. After this, players can pick any court they currently have, and they can select the number of games and sets. Then, the match starts. Chance Shots cannot be turned off, unlike Power Shots from Mario Power Tennis.

Another regular feature of the Mario Tennis series, the Special Games, is also present in Mario Tennis Open. These games, like the name says, have special rules and features that differ from normal gameplay. Some of these games bear a very strong resemblance to the Special Games in the previous Mario Tennis titles. However, unlike in the previous Mario Tennis games, Ring Shot is included within the Special Games rather than as another option for exhibition matches. Each Special Game has four difficulties, which are named according to the Special Game. Other than Super Mario Tennis, the last difficulty is a challenge that tests how much a player can do before running out of tries.

Players can unlock characters by clearing Level 3 of each Special Game. Players can unlock outfits for their Mii if they meet the requirements for unlocking them in level 4.

This game is similar to Ring Shot from Mario Tennis. The player must win by hitting the ball through rings that appear over the net. Multiple rings appear, each decreasing in point value as they get bigger. The game is over when time runs out or the goal is achieved.

In this game, the player must hit the ball onto a wall with Super Mario Bros. levels scrolling on it. Hitting items, enemies, blocks, and coins will extend the time, and hitting the flagpole will finish the level. The game is over when the level is completed or when all lives are lost.

The player must rally a ball with a Luma without making the ball fall into the Black Hole. There are Shrinking Tiles which disappear when the ball bounces on them. The game is over when the three balls are lost or the goal is achieved.

There are a total of 25 playable characters in Mario Tennis Open. Four of these are unlocked by completing level 3 of their respective Special Games, while Metal Mario and other-colored Yoshis are unlocked by scanning QR codes. Additionally, each character will be in one of the six player classes available in the game, excluding the Miis as they can be customized.

Characters are split into six categories of types depending on their stats.

All-Around characters do not have any major advantages or disadvantages.

Technique characters have better ball control, often at the expense of power.

Speed characters move quickly around the court, often at the expense of power.

Power characters have faster, stronger strokes and serves, but are often not very agile characters.

Defense characters are large or long characters with a better ball reach. They are not very agile characters.

Tricky characters have highly curving shots, making it harder for the opponent to predict shots. They are not very powerful characters.

Yoshi is a well-rounded speed character. He has fewer disadvantages than other speed characters, but has fewer advantages over other speed characters as well, such as having middle-ground speed, power, and reach among the speed characters.

Bowser is the most powerful starter power character. His strokes and serves are very quick and powerful, and Bowser has above average reach. However, Bowser's agility and lunges are the lowest of the starter characters.

Boo has the most highly curving shots in the game, though it is not a powerful character and it does not have a good reach. Its lunge is short-ranged, but very quick, a change over its slower, longer-ranged lunge from Mario Power Tennis.

Miis are custom characters whose stats are determined by the gear they are equipped with. Without any gear, they are well-rounded characters who have no distinct advantages or disadvantages. The only stat that cannot be altered for the Mii is its reach, as players cannot increase or decrease sizes of Miis in this game, but they can do so in the Mii Maker functionality of the Nintendo 3DS.

Baby Mario is the fastest of all speed characters, has the longest lunge, and is the most powerful of speed characters. Due to his small size, his reach ranks among the worst in the game and his ball control is average.

Dry Bowser is a powerful Defense character who is not as powerful as the power characters, but has better ball control over them. Dry Bowser also has better volley and slice than Bowser, but worse stroke and topspin.

The only Yoshi, and by extension, only QR downloadable character to have an All-Around play style. This makes Black Yoshi the most balanced of the Yoshis, and he has no significant advantages or disadvantages.

The only Yoshi, and by extension, the only QR downloadable character with a Tricky play style. Compared to other Yoshis, his shots have a high curve, making them more unpredictable, but he also has lower power.

The Yoshi costume that the player can unlock by scanning the appropriate QR code.

In Europe, a Yoshi QR Chase was set up in 30 participating ASDA stores, and in participating EB Games and JB Hi-Fi stores in Australia and New Zealand. This special QR event allowed consumers to scan the code via the game to unlock certain Yoshis to play as. All countries where the game has been released have all QR codes in regards to the colored Yoshis.

Although not part of the Yoshi Hunt, a Yoshi costume (pictured right) can also be unlocked for the player's Mii by scanning a specific QR Code.

Gear may be bought for the player's Mii with coins that that are earned by playing Special Games. They can be bought at the Clubhouse for a certain amount of coins each and alter the Mii's statistics. Costumes can also be unlocked by meeting certain conditions, such as giving characters a star rank.

Mario Tennis Open received mixed to positive reviews from critics, with aggregate websites Metacritic and GameRankings giving the game a 69 based on 58 reviews[1] and 69.26% based on 38 reviews.[2] Critics often cite that while the title is considered a solid title, they lament that it plays and feels too similarly to previous titles in the series, with Chance Shots not greatly improving or changing the gameplay up to make the title stand out, and the game was overall a disappointment. Reception towards the Mii customization was mixed; the overall customization was praised, but the pie-chart system of viewing stats has a universal negative reaction. Some critics complain that Miis receive too much focus compared to the Mario series characters as well. Praise has been given to the Special Games, however, especially the Super Mario Tennis Special Game.

Eric L. Patterson of Electronic Gaming Monthly has felt that while the title is a solid one for any Mario Tennis fan, it is not a great Mario Tennis title, and gave the game a 7 out of 10.[3] Richard George of IGN has concluded that the game is "Okay", acknowledging that, "Camelot’s decade of tennis experience means they understand how to make the sport engaging and addicting...Yet Mario Tennis Open struggles in just about every other regard.", where the score is a 6.5 out of 10.[4] Griffin McElroy of Polygon has lambasted the Chance Shots system in his review, calling out the over-reliance on them and their random spawning.[5] On a more positive note, Matt Helgeson from Game Informer has given the game an 8/10, appreciating how Mario Tennis Open goes back to the basics due to him disliking the Power Shots feature of Mario Power Tennis, and that the game introduces online to the series.[6]

"Stacked up, these deficiencies overwhelm what is, at its core, a great game. It’s a shame just about everything Mario Tennis Open attempts to add on top of that is remarkably unworthy of its lineage."

"It might not hold a candle to the handheld Mario Tennis games in the Game Boy lineage, but Mario Tennis Open is a still great game that is sadly hampered by a small feature set and harebrained online."

"What little content is here doesn't reach the heights that it should. I can't remember the last time an otherwise superb game was betrayed so completely by a single mechanic. Instead of making the whole of Mario Tennis Open about strategically countering your opponent's volleys, a system which is fully and brilliantly implemented, Camelot has made it an afterthought. It's something you do to stay alive while you wait for the stars to align."

"I enjoyed Power Tennis (both times), but the balance-breaking power shots were far too vital to winning. In response, Camelot has scaled back the wackiness with Open, which translates to a casual tennis game that plays it fairly straight."

"Mario Tennis Open’s single player mode is fun for a few hours, but it will ultimately leave you wanting more. Thankfully, the multiplayer mode with its online functionality will keep you coming back well after you’ve plowed through the main course. Mushroom Kingdom tennis vets will miss the career mode, but anyone just looking for a solid competitive game that’s best enjoyed in short bursts will be well taken care of."

"Mario Tennis Open impresses and disappoints, unfortunately, proving to not be the out-and-out champion many were expecting, yet still managing to offer enough familiar fun to engage fans of old and newcomers alike. Brace yourself for a brief single-player mode and remove all thoughts of serious tennis from your mind and it will not be too much of a let-down."

Super Mario Bros.: A remix of the overworld music from this game is heard in the tune that plays when the trophies for the Mushroom, Flower, and Banana Cups are displayed (after the scenes where the character is shown with his/her trophy). Part of the melody of this tune is heard in the music for the title screen and credits, and during exhibition matches in Mario Stadium. The Special Game Super Mario Tennis is heavily based on this game. The overworld, underground, and castle themes from this game are featured, as several levels are replicated (albeit with minor revisions). The Super Mushroom, Fire Flower and Super Star also appear in Super Mario Tennis.

Mario Tennis (Nintendo 64): The special game Ink Showdown is based off Piranha Challenge. Also, the game, break, set, and match point themes for the Star Open tournaments are covers from those in this game. Baby Mario also returns as a playable character with the exact stats and some recycled voice clips from this game. The Toad, Birdo, and Shy Guy suits can also be earned, along with their rackets, which is a reference to their being playable characters in the original Mario Tennis. The equipment also gives the player's Mii similar stats that they had in this game too.

Mario Tennis (Game Boy Color): The theme music that plays during the Set or Match Point of the aforementioned game is a cover and sampled for the Star Open Set Point in this game.

Wario Land 4: The pyramid featured in this game appears at the front of the Wario Dunes court.

Super Mario Sunshine: A small bit of Bowser's battle theme from this game (which was used as the theme for Bowser's Castle in the previous game) is interpolated in the new Bowser's Castle theme.

Mario Power Tennis: Galaxy Rally plays similarly to Gooper Blooper Volley. Mario Stadium is very similar to the Peach Dome. The rackets and costumes of Koopa Troopa, Petey Piranha and Wiggler can also be earned, which recalls their being playable characters in this game. Also, the character and announcer voice clips, and characters' actions in the menu and actual match, are reused from this game. But this time, the characters and announcer adopt new schemes, which are also carried on to later games:

The characters perform their winning and losing actions not just after the match, but after games and sets as well.

The announcer does not say "final set" when the match has only one set.

The name of the serving character is no longer mentioned.

The announcer says "server" or "receiver", instead of the character's (or characters', for doubles) name, to indicate who won the game, set, or match, or has the advantage after a deuce.

Mario Kart Wii: Mushroom Valley takes place in Mushroom Gorge, as the track itself can be seen below the court. The court takes place on a Mushroom, which brings back the aspect of the red mushrooms being very bouncy. Also, several voice clips are reused from this game.

Rosalina was intended to appear as a playable character, but was replaced by Luma due to the sheer amount of time it would take to model her character.[8] She would later be included as a playable character in the next Mario Tennis installment, Mario Tennis: Ultra Smash.